internecine: [17] Etymologically, internecine denotes ‘attended by great slaughter’. Its modern connotations of ‘conflict within a group’, which can be traced back to the 18th century (Dr Johnson in his Dictionary 1755 defines it as ‘endeavouring mutual destruction’), presumably arise from the standard interpretation of inter- as ‘among, between’. But in fact in the case of internecine it was originally used simply as an intensive prefix.
The word was borrowed from Latin internecīnus, a derivative of internecāre ‘slaughter, exterminate’. This was a compound verb formed with the intensive inter- from necāre ‘kill’ (a relative of English necromancy and pernicious). => necromancy, pernicious[internecine etymology, internecine origin, 英语词源]